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TERREOHMETRY 


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By  Ethan 


LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 


GIFT  OF 


Manual  of 
Terreohmetry 

m 

By  Ethan  Scheidler,  Terreohmetric  Engineer 


, 
or  THE 

UNIVERSITY  } 

OF  J 

.*'  -"  cy.^,-,>--'"! 


Copyright  1905,  by  ETHAN  SCHEIDLEB 


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CONTENTS 

PREFACE 

NOTE  ON  PATENTS 

CHAPTER  I. 

Sec.  I  Definition 

"    2  Theory  of  Earth  Resistance 

"    3  Resistance  of  Rocks 

"    4  Relation  of  Distance  to  Resistance 

CHAPTER  II.  Description  of  Apparatus 

Sec.  i  List  of  Apparatus 

"    2  Description  of  the  Terreohmeter 

"    3  Other  Apparatus 

"    4  Making  Repairs 

CHAPTER  III.  Measurement  of  Earth  Resistance 
Sec.  i     General  Principles 

«    2     Method  of  Securing  Correft  Measurements 
"    3     Difficult  Conditions 
"    4     Underground  Contacts 
"    5     The  Minimum  Number  of  Electrodes 

CHAPTER  IV.  Technical  Description  of  a    Terreohmetric 

Survey 

Sec.  i     Establishing  Stations 
"    2     Position  of  the  Instrument 
"    3     Area  of  Earth  Contacts  and  their  Distance 

Apart 

"    4     Protection  of  Stations 
"    5     Handling  Wire 
"    6    The  Switchboard 
"    7    The  Plat  and  Report 
"    8    Field  Notes 


CHAPTER  V.  Application  of  the  Terreohmetric  Art 

Sec.  i  General  Principles 

"  2  Arcs,  or  the  Method  of  Locating  Ores  Oc- 
curring in  More  or  Less  Horizontal  Beds 
or  Irregular  Deposits 

ft    3  Parallels,  or  the  Method  of  Locating  Ledges 

"    4  Tracing  Extended  Deposits 

tf     5  Determining  Depth 

"    6  Comparison  of  Different  Portions  of  Deposit 

"    7  Size  of  Deposit 

"    8  Faults 

"    9  Complicated  Problems 

"10  Placer 

"IT  Massive  Minerals 

"12  Gold 

"13  Buried  Treasure 

"  14  Problems  Impossible  to  Solve 

"15  Country  Rocks  of  Low  Resistance 

"  1 6  How  to  Make  Deductions 


-   OFT*' 

PREFACE!  gi"v 


In  order  to  have  a  correct  understanding  of 
this  Manual  it  is  necessary  that  the  reader  shall 
be  possessed  of  a  fair  working  knowledge  of  (1) 
the  department  of  electricity  which  treats  of  re- 
sistance, (2)  civil  engineering,  and  (3)  the 
geology  of  ore  deposits.^ 

*Note. — The  necessary  knowledge  of  electricity 
may  be  obtained  by  a  careful  study  of  the  chapters 
on  measurement  of  resistance  in  any  standard  text- 
book. 

Some  practical  experience  in  ordinary  surveying 
is  very  helpful.  If  the  engineer  is  without  this  it 
will  be  wise  for  him  to  secure  the  services  of  an 
assistant  who  has  such  experience. 

Any  of  the  standard  works  on  geology  may  be 
used  in  preparation  for  the  practice  of  this  art. 
Practical  experience  in  prospecting  and  mining  also 
counts  for  much. 

No  matter  how  much  or  how  little  preparatory 
education  and  experience  engineers  may  have, 
there  is  one  rule  that  applies  to  all,  and  must  be  re- 
membered as  essential  to  success  in  terreohmetric 
surveying,  namely,  all  preconceived  notions, 
theories  and  opinions  in  regard  to  the  existence,  lo- 
cation or  character  of  mineral  deposits  in  the  area 
to  be  surveyed  must  be  disregarded.  Terreohmetry 
is  an  accurate  method  of  observing  facts,  and  the 
mind  of  the  engineer  must  be  free  from  speculations 
in  order  that  he  shall  make  the  survey  correctly. 
After  the  work  is  completed,  having  the  facts  es- 
tablished thereby  as  additional  basis  to  what  was 
before  known,  he  may  construct  new  theories  if  he 
pleases  in  regard  to  what  is  still  undetermined. 


Terreohmetry  is  a  new  science  the  develop- 
ment of  which  was  commence*!  soon  after  the 
invention  of  the  terreohmeter  in  1900.  The  au- 
thor has  conducted  numerous  experiments  under 
a  great  variety  of  condition*:  to  determine  the 
fundamental  principles  of  the  science.  All  was 
theory  to  begin  with,  based  only  on  the  analogy 
of  other  sciences.  The  result  was  that  some  no- 
tions had  to  be  discarded ;  that  facts  new  to  the 
scientific  world  have  been  established  and  prin- 
ciples deduced  therefrom. 

Herein  the  attempt  has  been  made  to  set  forth 
in  practical  form  the  knowledge  derived  both 
from  such  experiments  an'd  the  work  of  above 
forty  actual  surveys. 

Let  it  be  recognized  that  the  science  is  in  its 
infancy  and  let  every  engineer  who  undertakes 
to  practice  the  terreohmetric  art  become  a  stu- 
dent of  it.  Wider  experience  will  surely  have  its 
fruit  in  a  steady  accumulation  of  valuable  in- 
formation. There  are  a  thousand  conditions  yet 
to  be  tested, — and  it  has  been  th.e  author's  ex- 
perience that  each  new  condition  means  a  new 
problem  to  be  solved.  But  having  the  founda- 
tion already  laid,  the  rest  is  comparatively  easy. 

Any  who  may  desire  further  instruction  or  ad- 
vice on  the  practice  of  terreohmetry  should  cor- 
respond with  the  author.  Address  South  Pasa- 
dena, California. 


NOTE  ON  PATENTS 

The  terreohmeter  is  covered  by  patents  issued 
to  the  inventor,  Fred  Harvey  Brown,  and  can- 
not legally  be  used  for -surveying  except  under 
rights  acquired  originally  from  the  patentee. 


CHAPTER  I. 

Sec.  1.  Definition.  Terreohmetry  is  the  sci- 
ence of  securing  and  comparing  the  electrical  re- 
sistances of  different  earth-sections  in  the  same 
vicinity  for  the  purpose  of  determining  whether 
bodies  of  mineral  or  mineral-bearing  ores  exist 
therein,  their  exact  location,  and  other  facts  con- 
cerning them. 

Sec.  2.  Theory  of  Earth  Resistance.  The  re- 
sistance of  massive  bodies  has  been  but  little  in- 
vestigated, that  of  the  earth  not  at  all  outside 
the  terreohmetric  art.  In  some  authorities  it  is 
stated  that  the  earth  has  no  resistance ;  in  others, 
that  it  cannot  be  measured.  The  latter  was  a 
fact  until  the  invention  of  the  terreohmeter.  The 
former  statement  is  a  mistaken  surmise,  arising 
from  necessarily  insufficient  investigation  of  the 
problem. 

This  field  of  study  is  exceedingly  interesting- 
all  the  more  because  it  is  new — and  bids  fair  to 
produce  a  wealth  of  useful  knowledge. 

In  measuring  the  resistance  of  massive  bodies, 
the  fundamental  principle  of  electricity  still  has 
first  place;— the  current  seeks  the  path  of  least 
resistance.  The  resistance  of  a  lineal  conductor 
decreases  as  the  diameter  increases.  Therefore 
in  traversing  a  massive  conductor  like  the  earth, 
the  current  utilizes  a  portion  of  large  sectional 
area.  If  the  earth-mass,  the  resistance  of  which 
is  measured,  be  perfectly  homogeneous  with  re- 
gard to  its  conducting  power,  it  is  supposed 
that  the  current  disperses  and  ramifies  through 
.it  somewhat  in  the  manner  that  the  brush  dis- 


charge  from  a  spark  coil  does  through  the  air, 
and  that  the  diameter  of  the  sectional  area  of 
earth  section  utilized  as  a  conductor  by  the  cur- 
rent varies  as  the  conductivity  of  the  material 
may  be  better  or  worse. 

In  other  words,  the  conductivity  of  a  section 
of  earth  is  dependent  upon  two  factors,  namely, 
the  mass  of  material  utilized  as  a  conductor  and 
the  relative*  conductivity  of  the  same. 

The  former  also  is  dependent  upon  the  latter 
in  some  ratio.  The  theory  is  incomplete  on  this 
point. 

In  practice  it  is  found  that  the  presence  of  low- 
resistance  minerals  in  an  earth  section  reduces 
the  resistance  thereof  in  a  ratio  much  larger 
than  would  be  supposed  considering  only  the 
mass  of  mineral  contained  therein.  This  is  true 
whether  the  mineral  be  in  fine  particles  diffused 
through  the  mass  or  in  lumps. 

Nature  does  not  often  furnish  the  ideal  condi- 
tion of  homogeneity  with  regard  to  conduc- 
tivity, consequently  a  comparative  study  of  the 
relative  resistances  of  the  different  earth  forma-, 
tions  covered  by  each  survey  is  a  wise  precau- 
tion. 

Sec.  3.  Resistance  of  Rocks.  Not  sufficient 
data  has  yet  been  collected  for  the  compilation 
of  tables  showing  the  actual  resistance  of  dif- 


*Note. — The  term  ''specific  conductivity"  would  be 
here  used,  but  it  is  impossible,  as  not  sufficient  re- 
liable data  has  been  collected  for  the  selection  of  a 
standard  of  conductivity  for  massive  earth  forma- 
tions and  the  creation  of  a  table  of  results  with  ref- 
erence thereto. 


ferent  kinds  of  rocks  in  massive  form,  but  ex- 
perience so  far  tends  to  establish  the  fact  that 
all  the  crystalline  rocks  offer  approximately  the 
same  resistance.  Compared  with  them  the  igne- 
ous rocks  are  much  poorer  conductors  and  the 
secondary  and  recent  rocks  are  considerably  bet- 
ter conductors. 

The  absolute  resistance  of  rocks  is  not,  how- 
ever, a  question  of  first  importance  in  terreohme- 
try,  as  that  art  requires  only  the  determination 
of  relative  resistances  and  the  location  of  bodies 
of  low- resistance  material  encased  in  masses  of 
comparatively  high-resistance  material.  Usually 
the  difference  of  resistance  between  the  material 
it  is  desired  to  locate  and  the  surrounding  rock 
is  so  great  that  any  variations  likely  to  be  found 
in  either  are  not  sufficient  to  interfere  appre- 
ciably with  the  location  of  the  former.  Most 
earth  formations  that  constitute  country  rocks 
are  exceedingly  poor  conductors  of  electricity  as 
compared  with  mineral  bearing  ores. 

Sec.  4.  Relation  of  Distance  to  Resistance. 
Resistance  increases  with  the  length  of  earth- 
section  measured,  but  the  exact  ratio  of  increase 
has  not  been  established.  Increase  of  resistance, 
however,  is  small  as  compared  with  increase  of 
length  of  earth-section. 

CHAPTER  II. 

DESCRIPTION  OF  APPARATUS. 

Sec.  1.  List  of  Apparatus.  The  necessary 
apparatus  for  ordinary  work*  consists  of:  the 

a 


terreohmeter  (otherwise  called  ''Brown's  Elec- 
tro-Geodetic Mineral  Finder"),  with  tripod; 
three  dozen  one-half  inch,  hexagon,  spring-brass 
rods,  30  inches  long,  pointed  at  one  end;  three 
one-half  inch  round  tool-steel  rods,  36  inches 
long,  pointed  one  end;  two  jacks  for  pulling  the 
rods  (see  fig.  4.) ;  three  dozen  spring  clamps  for 
attaching  the  wire  to  the  rods ;  one  3y2  lb.  ham- 
mer; one-half  dozen  spools  (about  4000  feet)  No. 
18  annunciator  wire;  one-half  dozen  field  reels 
which  must  be  made  especially  for  this  work 
(see  fig.  5.) ;  and  sufficient  canteens  or  other  re- 
ceptacles for  conveying  water  to  be  used  in  mak- 
ing earth-contacts. 


FlG.l.riRST  PROCESS 


FI6.2. 

SECOND  PROCESS 


*Note. — The  amount  of  apparatus  necessary  de- 
pends upon  the  kind  of  work  that  is  to  be  done.  The 
list  given  is  that  which  the  author  found  necessary 
in  surveying  large  mining  properties  on  the  Pacific 
Coast. 

11 


r 
*- 
i^ 


•—  H.? 


FI6.5. 

THE  SCALE 


! * 


5.  Description  of 
the  Terreohmeter.  The 
terreohmeter  is  an  adapta- 
tion of  the  Wheatstone 
Bridge.  (See  Fig.  1.)  The 
primary  circuit  includes 
battery,  switch,  interrupt- 
er, and  the  primary  of  a 
small  induction  coil.  The 
secondary  circuit  traverses 
the  arms  of  the  bridge,  one 
of  which  consists  of  a  short 
piece  of  .  fine  nickel-steel 
(or  other  high-resistance) 
wire  laid  on  a  scale  cali- 
brated to  read  from  1  ohm 
theoretically  to  infinity,  but 
practically  to  about  10,000 
ohms  with  ample  accuracy. 
( See  Fig.  3,  which  is  one- 
half  actual  size. )  This  scale 
wire  should  have  a  resist- 
ance of  not  less  than  7  or  8 
ohms.  The  other  arm  of 
the  bridge  includes  the 
"earth"  and  a  coil  of 
known  resistance.  In  some 
instruments  the  latter  is 
250  ohms;  in  others  there 
are  two  coils;  of  100  and 
1000  ohms  respectively, 
either  of  which  may  be 
used  as  desired  by  ehang- 

12 


ing  a  plug.  Fig.  3  shows  the  scale  of  the  latter 
form  of  instrument. 

The  arms  are  "  bridged "  by  a  telephone  re- 
ceiver in  circuit  with  a  stylus,  connected  by 
flexible  cord.  The  stylus  is  held  in  the  hand  and 
brought  into  contact  with  the  scale-wire,  while 
the  receiver  is  held  to  the  ear.  A  sound  is  heard 
in  the  receiver,  which  decreases  in  volume  as 
the  stylus  is  moved  along  the  jscale,  until  a  point 
of  silence  is  reached.  Beyond  this  point  the 
soun'd  is  again  heard.  The  figures  on  the  scale 
at  the  point  of  silence  indicate  the  number  of 
ohms  resistance  of  the  earth  section  or  other  ob- 
ject measured. 

The  terreohmeter  is  small  that  its  weight  may 
not  be  burdensome.  The  strength  of  current 
has  no  effect  on  the  accuracy  of  the  instrument. 
As  long  as  there  isi  sufficient  current  to  cause 
the  buzzer  to  vibrate,  measurements  may  be 
taken  and  are  the  same  as  though  a  powerful  bat- 
tery were  used. 

There  are  two  processes.  One  form  of  appara- 
tus (see  Fig.  1)  uses1  an  alternating  current.  In 
the  other  (see  Fig.  2)  the  induction  coil  is  re- 
placed by  an  electro-magnetic  coil  '(having  pri- 
mary winding  only)  and  the  bridge  is  traversed 
not  by  an  alternating  current,  but  by  an  inter- 
rupted extra-direct  current.  Either  form  of  ap- 
paratus may  be  used  with  equal  facility,  but  the 
second  is  simpler  and,  under  some  conditions, 
has  been  found  to  produce  a  more  decided  point 
of  silence  on  the  scale. 

All  connections  within  the  instrument  should 
be  soldered.  The  battery  should  consist  of  two 
or  three  small  dry  cells.  13 


The  switch  must  be  kept  open  except  when  the 
current  is  actually  employed  in  measuring  resist- 
ance ;  otherwise  the  battery  will  soon  become  ex- 
hausted. When  using  the  instrument  great  care 
should  be  exercised  to  touch  the  scale- wire  very 
lightly  with  the  stylus.  Otherwise  the  wire  will 
soon  become  so  much  abraded  that  the  accuracy 
of  the  instrument  will  be  entirely  destroyed. 

A  heavy  camera  tripod  serves  very  well  as  a 
mount  for  the  instrument.  To  attach  same  use 
a  square  brass  plate,  with  suitably  threaded  cen- 
ter hole,  screwed  to  the  bottom  of  the  instrument 
box. 

When  the  battery  and  buzzer  are  in  a  box 
separate  from  the  rest  of  the  apparatus,  this 
second  box  should  be  strapped  to  one  of  the 
legs  of  the  tripod.  When  the  battery  is  contained 
in  the  instrument  box,  the  buzzer  should  be  car- 
ried in  the  pocket,  suitably  wrapped  to  deaden 
its  sound,  and  connected  in  for  each  reading  of 
measurement  by  plugs  or  snap  connectors.  In 
this  case  no  switch  is  needed,  for  the  removal  of 
the  buzzer  connections  breaks  the  battery  circuit. 


w- 

y 

~Z?^ 

•  ,  1 

i 

FIG.4.    RODJACK 

• 

14 


Sec.  3.  Other  Apparatus.  The  rod-jack  is 
made  of  %  nlch  g^8  pipe.  A  piece  of  solid  steel 
rod  is  welded  in  at  the  end  of  the  handle,  split 
and  bored  to  receive  %  inch  bolts.  (See  Fig.  4.) 
The  link  which  engages  the  rod  is  of  forged 
steel. 

For  the  spring  clamps  mentioned  in  Sec.  1, 
Bulldog  paper  clamps  that  can  be  purchased  at 
any  large  stationery  store,  have  been  used  and 
found  to  serve  very  well  though  they  are  much 
wider  than  necessary.  The  essential  part  of  this 
device  is  a  strong  spring  to  hold  the, wire  firmly 
in  contact  with  the  rod. 


FIG.  5       FIELD  REEL 


The  field  reels  (see  Fig.  5),  are  made  prin- 
cipally of  wood.  The  side  pieces  are  constructed 
in  the  same  manner  as  chair  bottoms.  Brass  axles 
are  fitted  neatly  into  bearings  of  the  same  metal. 

Other  devices  for  making  contact  with  the 
earth  have  been  used,  such  as,  brass  cylinders, 
brass  strips  made  in  the  form  of  a  gridiron,  brass 
plates,  steel  rods.  The  brass  rods  described  have, 
however,  been  found  by  far  the  most  practicable 
form  of  electrode  to  use  in  actual  work. 


15 


Of  THE 

UNIVERSITY 


or 


The  steel  rods  are  used  to  make  holes  in  which 
brass  rods  are  placed  and  their  use  saves  the  lat- 
ter from  destruction  by  excessive  hammering. 

Sec.  4.  Making  Repairs.  Accidents  frequent- 
ly happen  to  the  apparatus.  The  following  hints, 
if  noted,  will  save  time  in  repairing  same. 

When  the  battery  cells  become  polarized  they 
may  be  renewed  by  making  an  opening  in  the 
top  with  a  nail  or  knife  blade  and  pouring  in  as 
much  water  as  the  cell  will  absorb  in  five  or  ten 
minutes.  Test  the  cells  separately  with  the  buz- 
zer. A  "live"  cell  will  not  send  a  current 
through  a  "dead"  one. 

If  the  point  of  silence  occurs  unexpectedly  at 
the  zero  end  of  the  scale,  the  chances  are  the  two 
ground  connections  are  crossed  so  that  the  earth 
is  short  circuited.  If  the  point  of  silence  ap- 
pears at  the  opposite  end  of  the  scale,  one  of 
the  ground  connections  is  broken. 

When  no  sound  can  be  heard  in  the  receiver, 
though  the  buzzer  is  known  to  be  vibrating,  some- 
thing is  wrong  with  the  receiver,  the  stylus,  or 
with  one  of  their  connecting  cords. 

Always  keep  the  battery  and  buzzer  connec- 
tions turned  up  tight.  Always  see  that  the  in- 
sulation is  removed  from  a  wire  where  it  is  placed 
in  contact  with  a  rod  and  that  the  surface  of  the 
rod  is  clean  at  that  point. 

CHAPTER  III. 
MEASUREMENT  OF  EARTH  RESISTANCE. 

Sec.  1.  General  Principles.  It  is  of  primary 
necessity  that  each  measurement  of  resistance  (or 

16 


"observation,"  as  it  is  sometimes  called)  shall 
be  taken  with  as  near  an  approximation  to  abso- 
lute accuracy  as  the  conditions  render  practic- 
able. 

If  a  stream  of  water  is  to  pass  freely  from  a 
flume  into  a  field  the  gateway  must  be  of  such 
size  and  shape  that  it  offers  no  obstruction  to  the 
water.  This  is  fairly  analogous  to  the  process 
of  constructing  a  circuit  for  an  electric  current, 
partly  of  wire  and  partly  of  a  section  of  earth. 
The  rods  are  the  gate  by  which  it  must  pass  into 
the  earth  and  must  be  of  such  number  and  so 
placed  in  contact  with  earth  particles  that  there 
is  the  least  obstruction  possible  to  the  free 
passage  of  the  current. 

Some  conditions  (a  moist,  compact,  clay  soil, 
for  instance)  render  this  such  an  easy  matter 
that  one  rod  inserted  to  a  depth  of  18  inches 
gives  practically  a  perfect  contact.* 

When  the  surface  soil  is  composed  of  loose-ly- 
ing gravel  or  roughly  broken  rock  mixed  with 
more  or  less  fine  material,  the  difficulty  is  in- 
creased and  great  care  and  watchfulness  on  the 
part  of  the  engineer  are  necessary  in  order  that 
the  work  may  be  perfectly  reliable.  This  is  true 
especially  where  a  variety  of  conditions  are  met 
with  in  the  area  surveyed.  If  good  contacts  are 
made  where  it  is  easy  to  make  them,  and  at  other 
points  on  the  same  survey  the  contacts  have  a 
variety  of  values  as  the  conditions  are  more  or 

*Note. — A  perfect  contact  is  one  which  offers  no 
resistance.  Perfect  contacts  must  be  made  at  both 
ends  of  an  earth  section  in  order  to  secure  an  abso- 
lutely accurate  measurement  of  earth  resistance. 

17 


less  difficult,  the  work  will  be  worthless  as  far 
as  deductions  regarding  the  location  of  mineral 
are  concerned. 

An  "indicated"  measurement  of  earth  resist- 
ance always  consists  of  a  sum  of  three  resist- 
ances, namely,  the  resistance  of  the  two  earth 
contacts  and  that  of  the  earth  section. 

An  "absolutely  accurate"  measurement  is  one 
which  is  made  to  show  the  resistance  of  the 
earth  section  alone.  An  indicated  measurement 
should  not  be  more  than  five  per  cent  greater 
than  the  same  if  taken  with  absolute  accuracy. 

This  should  be  carefully  studied  as  it  is  a  very 
important  point.  The  above  is  a  description  of 
the  greatest  and  most  common  error  committed 
by  those  who  have  attempted  to  practice  this  art. 
The  engineer  should  therefore  be  sure  that  he 
understands  thoroughly  the  following  section. 

Sec.  2.  Method  of  Securing  Correct  Measure- 
ments. The  rule  which  must  be  followed  is: 
MAKE  EVERY  CONTACT  PRACTICALLY  PERFECT. 
This  must  be  done,  no  matter  how  difficult  the 
task,  or  the  work  will  not  be  worth  recording.* 

The  following  is  the  method  by  which  measure- 

*Note. — It  will  not  do  to  use  for  earth-contacts  one, 
or  two,  or  even  three,  electrodes  driven  to  the  same 
depth  at  every  contact  point  on  a  survey,  and  as- 
sume that  by  this  means  conditions  are  equalized 
and  the  results  therefore  rendered  reliable.  This 
is  an  error  which  must  be  carefully  avoided.  On  a 
survey  made  in  this  manner  some  contacts  are  per- 
fect; others  have  a  value  equal  to  the  resistance  of 
the  earth  section  measured;  others  have  a  value  ten 
or  twenty  times  that  of  the  earth  section  measured. 
The  record  of  work  done  in  this  manner  is  mani- 
festly worth  little. 

18 


ments  of  sufficient  accuracy  may  be  secured  any- 
where: at  one  end  of  the  earth  section 
to  be  measured  insert  a  steel  rod  to  any 
depth  to  which  it  can  be  driven  with 
the  use  of  moderate  force,  taking  care 
to  leave  sufficient  length  of  rod  projecting  so 
that  the  jack  can  be  used  to  draw  it  out.  If  it  is 
held  so  firmly  that  there  is  danger  of  breaking 
the  rod  or  jack,  use  two  jacks,  set  on  opposite 
sides  of  the  rod.  Rods  should  never  be  driven 
with  such  force  as  to  be  bent  either  in  driving  or 
drawing.  Unless  the  ground  is  already  very  wet, 
fill  the  hole  with  water.  Insert  a  brass  rod  to 
the  bottom  of  the  hole — no  more,  no  less.  With 
one  of  the  spring  clamps  attach  an  end  of  wire 
to  the  electrode,  which  now  becomes  an  earth- 
contact.* 

Unreel  wire  to  the  other  end  of  the  earth  sec- 
tion. In  the  same  manner  as  before  place  an 
electrode  in  position.  Now  set  up  the  instru- 
ment and  attach  the  line-wire  (that  which  con- 
nects with  the  distant  contact)  to  one  of  the  bind- 
ing screws  intended  for  the  earth  connections. 
It  is  not  necessary  to  cut  the  wire  in  order  to  do 
this ;  simply  scrape  off  the  insulating  material  for 

*Note. — A  brass  "rod"  becomes  an  "electrode" 
when  it  is  properly  placed  in  the  earth  and  con- 
nected so  as  to  form  a  portion  of  an  electric  circuit 
of  which  a  wire  and  an  earth  section  are  also  por- 
tions. 

An  "earth-contact"  is  constituted  of  one  electrode 
or  a  group  of  electrodes  connected  together  so  as  to 
form  one  gateway  for  the  passage  of  the  current. 
There  must  of  necessity  be  two  earth-contacts  in 
every  complete  circuit  which  includes  an  earth  sec- 
tion. 

19 


the  distance  of  an  inch  and  slip  this  bare  portion 
of  wire  under  the  screw.  With  a  short  piece  of 
wire  connect  the  last  rod  set  with  the  other  bind- 
ing screw. 

Now  take  the  reading  and  record  the  resist- 
ance. Next,  place  another  electrode  in  position 
a  foot  or  so  from  the  last  one.  Connect  the  two 
last  electrodes  together  and  take  another  read- 
ing as  before,  except  that  now  the  last  two  elec- 
trodes together  form  one  earth-contact,  Note  the 
difference  between  the  readings. 

If  the  second  reading  is  a  material  reduction 
of  the  first,  continue  by  inserting  a  third  rod  in 
close  proximity  to  the  two  already  in  use  and 
connect  it  to  them.  Each  rod  is  inserted  to  any 
depth  that  can  be  attained  with  the  use  of  reason- 
able force,  without  reference  to  the  depth  to 
which  other  rods  are  driven.  Take  the  reading 
again  and  note  the  reduction.  Continue  thus  in- 
creasing the  number  of  electrodes  used  to  make 
the  earth-contact,  taking  the  reading  with  the 
addition  of  each  electrode,  until  the  reduction 
noted  on  the  addition  of  an  electrode  is  less  than 
one  per  cent  of  the  final  reading.  By  this  pro- 
cess a  contact  having  a  sufficient  degree  of  per- 
fection is  obtained  at  this  point.  But  the  first 
earth-contact  still  remains  to  be  corrected. 

Disconnect  the  line  wire  from  the  instrument 
and  attach  it  to  the  entire  earth-contact  just  com- 
pleted, in  place  of  the  short  connection,  which 
is  removed.  Eeturn  with  the  instrument  to  the 
first  earth-contact.  Disconnect  the  line  wire  from 
the  electrode  and  attach  it  to  the  instrument,  us- 
ing an  appropriate  binding  post.  Again  use  a 
20 


short  connecting  wire  from  the  instrument  to 
the  electrodes.  Repeat  the  last  reading  taken  at 
the  other  end  of  the  line.  Repeat  the  operation 
of  adding  electrodes  to  the  contact,  testing  it  af- 
ter the  addition  of  each  one  until  a  reduction  of 
less  than  one  per  cent  of  the  final  reading  is 
again  obtained.  Thisi  last  reading,  then,  is  suf- 
ficiently accurate  to  be  recorded  as  the  indicated 
resistance  between  the  points  of  contact. 

In  practical  work  the  conditions  usually  vary 
so  that  from  two  to  five  electrodes  are  required  to 
make  contacts.  The  following  is  a  typical  table 
of  results  from  sruch  work  as  that  described 
above : 


From  1 

f  electrode  at  ) 
1st  contact  to  j" 

.    /electrode  at) 
I  2nd  contact  / 

2000  ohms 

1 

do 

2 

do 

1700 

reduction    300 

1 

do 

3 

do 

1600 

100 

1 

do 

4 

do 

1570 

30 

1 

do 

5 

do 

1565 

5 

2 

do 

5 

do 

1200 

365 

3 

do 

5 

do 

1100 

100 

4 

do 

5 

do 

1080 

20 

5 

do 

I 

do 

1075      *                       '                5 

It  will  be  noted  that  the  last  reduction,  5  ohms, 
is  less  than  one  per  cent  of  the  final  reading,  1075 
ohms.  A  further  addition  of  any  number  of 
electrodes  at  both  ends  of  the  line  will  not  pro- 
duce a  reduction  of  the  reading  five  per  cent. 

Sec.  3.  Difficult  Conditions.  Ground  that  is 
so  rocky  and  barren  of  soil  that  correct  contacts 
cannot  be  obtained  with,  say,  five  or  six  rods, 
should  be  avoided  if  possible.  If  the  nature  of 
the  survey  requires  that  contacts  be  made  in 
such  ground,  it  is  sometimes  necessary  to  drill 
holes  for  the  electrodes.  Ordinary  miners7  drills 
should  be  used  and  the  holes  filled  with  wet  clay 

21 


or  stiff  mud  made  of  the  finest  material  obtain- 
able, tamped  lightly.  The  rods  sihould  be  driven 
carefully,  one  in  each  hole  thus  prepared. 

Sec.  4.  Underground  contacts  in  soft  seams 
can  usually  be  made  with  two  or  three  electrodes. 
Where  drill-holes  in  hard  rock  must  be  used,  as 
many  as  ten  or  a  'dozen  electrodes  are  often  neces- 
sary to  make  a  contact,  ftn  such  cases  special 
care  must  be  used  to  see  that  an  ample  number 
of  electrodes  is  employed.  It  is  better  to  use  the 
time  in  securing  few  measurements  and  have 
them  perfectly  reliable  than  to  accumulate  a 
larger  amount  of  data  of  a  doubtful  nature. 

Sec.  5.  The  Minimum  Number  of  Electrodes. 
Surface  conditions  are  often  met  with,  which  ren- 
der it  an  easy  matter  to  make  contacts.  Depen- 
dence should  never  be  placed  on  one  electrode, 
however,  as  conditions  often  vary  unexpectedly. 
It  is  impossible  to  know  what  a  rod  penetrates 
beyond  the  first  inch  or  two  of  surface.  If  the 
material  happens  to  be  such  that  a  one-electrode 
contact  has  a  his-h  resistance,  the  second  electrode 
will  reveal  the  fact.  It  is  therefore  necessary  to 
follow  the  rule  of  always  using  at  least  two  elec- 
trodes at  each  contact. 

CHAPTER  IV. 

TECHNICAL  DESCRIPTION  OF    A    TERREOHMETRIC 
SURVEY. 

Sec.  1.  Establishing  Stations.  The  first  act 
in  a  survey  is  the  establishment  of  one  or  more 
"stations."  These  are  earth-contacts  which  are 
used  as  centers  from  which  many  measurements 
are  taken.  The  contact  resistance  of  stations  is 

22 


eliminated  by  the  test  described  in  Chapter  III, 
using  for  the  purpose,  in  the  case  of  the  first 
station,  a  line  wire  connecting  with  an  earth- 
contact  called  a  "  test-station, "  consisting  of  one 
or  two  electrodes  placed  at  a  distance  of  not 
less  than  100  feet.  This  method  may,  of  course, 
be  used  with  any  station1,  but  it  is  usually  the 
case  that  in  establishing  stations  subsequent  to 
the  first,  some  line  wire  by  which  the  necessary 
tests  may  be  made,  is  convenient  to  hand.  The 
electrodes  forming  a  station  must  not  be  disturb- 
ed in  any  way  while  in  use. 

"Line- wire"  or  merely  "line"  is  an  expres- 
sion arbitrarily  used  to  indicate  a  wire  which 
connects  with  a  station  at  a  distance,  and  distin- 
guishes it  from  the  short  wire,  or  "connection," 
which  extends  from  the  instrument  to  the  earth- 
contact  that  is  close  at  hand.  When  several 
lines  are  beinig  used,  connecting  to  various 
stations  in  different  parts  of  the  field,  they  are 
numbered  to  correspond  with  the  arbitrary  num- 
bers assigned  to  the  stations  to  which  they  are 
connected. 

Sec.  2.  Position  of  the  Instrument.  Always 
set  up  the  instrument  at  the  working  end  of  the 
line.  Never  set  up  the  instrument  at  a  station 
and  attempt  to  measure  a  set  of  resistances  pro- 
duced by  changing  the  distant  end  of  the  line 
to  successive  points  of  contact.  He  who  reads 
the  instrument  and  records  the  measurements 
must  be  near  enough  to  those  who  drive  the  rods 
to  observe  closely  the  conditions  they  are  meet- 
ing and  direct  their  work. 

Sec.  3.     A rea  of  Earth-Contacts  and  their  Dis~ 


23 


tance  Apart.  As  a  general  rule  the  electrodes 
used  for  each  earth-contact  may  be  distributed 
over  an  area  the  diameter  of  which  is  not  greater 
than  two  per  cent  of  the  length  of  earth-section 
measured.  Thus,  when  measuring  the  resistance 
of  an  earth  section  one  hundred  feet  in  length, 
the  electrodes,  which,  connected  together  pro- 
duce the  earth-contact  at  each  end,  may  be  dis- 
tributed over  an  area  of  ground  surface  not 
greater  than  two  feet  in  diameter.  When  the 
earth  section  is  a  thousand  feet  in  length,  the 
rods  may  be  distributed  over  an  area  twenty 
feet  in  diameter,  if  desired,  without  affecting 
the  accuracy  of  the  work. 

Contact  points  on  arcs  or  lines  (see  Figs.  6  and 
7)  should  seldom  be  placed  at  greater  distance 
than  25  or  30  feet  apart,  usually  much  less.  For 
economy  of  time,  this  distance  should,  of  course, 
be  as  great  as  may  safely  be  used.  No  rule  can 
be  given  that  will  apply  to  all  conditions.  The 
preliminary  measurements  on  each  survey  can 
often  be  planned  so  that  their  results  determine 
this  question — at  least  in  a  general  way — for  the 
conditions  there  found. 

Sec.  4.  Protection  of  Stations.  When  several 
stations  are  in  use  on  a  survey,  a  full  set  of  meas- 
urements between  them  should  be  taken  and  re- 
corded :  that  is,  1  to  2,  1  to  3,  1  to  4,  2  to  3,  2  to  4, 
3  to  4,  etc.  These  measurements  should  be  fre- 
quently repeated  during  the  progress  of  the  sur- 
vey to  make  sure  that  the  lines  and  stations  have 
not  been  disturbed.  Underground  stations  and 
any  that  may  be  in  remote  spots  on  the  surface 
may  thus  be  known  to  be  in  perfect  condition 

24 


though  not  seen  for  days  by  any  of  the  survey- 
ing party.  If  a  variation  is  discovered  the  dam- 
age must  be  repaired  before  the  station  can  be 
again  used. 

The  line-wire  attached  to  a  station  must  be 
fastened  to  some  secure  object  near  the  elec- 
trodes, so  that  in  case  of  sudden  strain  coming 
upon  the  line  it  will  not  be  jerked  loose  from  the 
rods. 

Sec.  5.  Handling  Wire.  Wire  is  unreeled  by 
grasping  the  reel-handle  (see  Fig  5)  in  one 
hand  and  holding  the  reel  so  that  it  will  turn 
easily  on  its  axle  as  the  one  holding  it  walks 
away.  The  thumb  of  the  hand  in  which  the  reel 
is  held  can  be  used  as  a  brake  if  there  is  a  ten- 
dency for  the  reel  to  turn  too  rapidly.  In  lay- 
ing wire  follow  the  most  practicable  route  of 
travel.  It  is  not  necessary  that  the  wire  should 
lie  in  a  straight  line  over  the  earth  section  to  be 
measured  and  it  is  not  economical  to  lay  it  by 
any  other  route  than  that  which  consumes  the 
least  time.  The  resistance  of  even  a  mile  of 
wire  is  so  small  that  it  need  not  be  considered 
in  a  survey. 

Wire  is  reeled  up  by  grasping  the  reel-handle 
with  one  hand  and  turning  the  reel  with  the 
other,  while  the  one  holding  it  follows  the  line 
by  which  the  wire  was  laid,  maintaining  suffi- 
cient tension  to  bind  the  wire  firmly  on  the  reel. 
Never  wind  up  the  wire  without  turning  the  reel 
as  this  twists  the  wire  and  causes  trouble  with 
kinks  when  it  is  used  again. 

A  wire  placed  in  a  shaft  should  be  fastened 
securely  at  each  level  to  prevent  the  whole  line 

25 


from  going  to  the  bottom  in  a  tangle  in  case  of 
accidental  breaking. 

Care  must  be  exercised  where  several  lines  lie 
near  or  across  each  other  to  keep  them  from 
touching  at  uninsulated  points. 

For  the  sake  of  convenience  and  economy  of 
time,  wires,  must  often  be  cut  and  spliced.  Use 
the  * '  telegraph  splice, ' '  made  as  follows :  remove 
the  insulation  from  each  wire  for  a  distance  of 
three  inches  from  the  end  and  scrape  the  metal 
bright  with  hack  of  the  knife  blade.  Bring 
the  ends  together  pointing  in  opposite  directions. 
Twist  them  together  at  a  point  about  three- 
quarters  of  an  inch  from  where  the  insulation 
begins.  Wrap  the  free  end  of  each  wire  tightly 
around  the  straight  portion  of  the  other.  A 
small  pair  of  pliers  is  needed  to  produce  a  neat 
and  satisfactory  splice.  Never  allow  carelessness 
in  the  performance  of  this  detail,  as  every  splice 
must  not  only  be  capable  of  sustaining  quite  a 
severe  strain,  but — what  is  more  important — 
it  must  fulfill  the  requirement  of  a  perfect  elec- 
trical contact. 

When'  the  plan  of  a  survey  is  formed,  wires 
may  often  be  laid  which  can  be  used  frequently, 
and  by  leaving  them  on  the  field  throughout  the 
survey,  even  though  their  entire  length  be  not 
in  use  continuously,  much  time  consuming  reel- 
ing up  and  unreeling  can  be  avoided. 

Sec.  6.  The  Switchboard.  When  several 
lines  are  in  use  a  small  switchboard  should  be  at- 
tached to  the  front  of  the  instrument.  It  con- 
sists simply  of  a  narrow  strip  of  wood,  in  which 
are  fastened  ten  binding  screws,  about 

26 


11/2  inches  apart,  having  no  connection  one  with 
another.  The  connection  to  the  earth-contact  is 
fastened  in  the  first  binding  screw.  The  oth- 
ers, or  as  many  as  may  be  required,  are  used  for 
the  line  wires,  which  are  unreeled  up  to  the  inv- 
strument  as  it  is  moved  from  point  to  point. 

Two  of  the  spring  clamps,  each  connected  to 
one  of  the  X  binding  posts  on  the  instrument 
by  a  piece  of  flexible  wire  about  two  feet  long, 
are  used  to  change  the  connections  quickly 
on  the  switchboard.  If  desired,  the  switch- 
board may  be  made  a  permanent  feature  of 
the  instrument  and  connections  placed  so  that 
the  ordinary  two  X  binding  posts  may  be  dis- 
pensed with. 

Sec.  7.  The  Plat  and  Report.  The  engineer 
must  make  a  plat  of  every  survey.  It  isi  an 
important  part  of  his  report.  More  than  that, 
it  is  an  indispensable  aid  to  himself  in  con- 
ducting the  survey.  Usually,  the  problem  to  be 
solved  can  be  defined  only  in  a  general  way  at 
first ;  but,  as  the  work  progresses  and  the  results 
are  added  to  the  plat  at  frequent  intervals,  the 
engineer  gains  the  knowledge  that  the  completed 
portion  of  the  survey  imparts,  the  problem  be- 
comes better  defined  as  its  solution  progresses, 
and  the  plan  matures  as  its  execution  proceeds. 

It  is  thus  impossible,  except  in  the  case  of 
very  simple  problems,  to  make  a  complete  plan 
of  a  survey  before  its  commencement.  The 
work  often  reveals  so  much  that  was  before  whol- 
ly unknown  and  perhaps  unsuspected,  that  un- 
foreseen features  of  the  problem  constantly  de- 
velop, often  necessitating  a  serious  modification  if 

27 


not  a  complete  change  of  the  first  plan.  Keep- 
ing the  measurements  entered  up  to  date  on  the 
plat  enables  the  engineer  to  follow  each  develop- 
ment of  the  work  and  plan  every  detail  with  that 
intelligent  understanding  which  is  necessary  to 
the  performance  of  the  survey  in  an  economical 
manner,  and  with  the  most  useful  results.  * 

When  a  plat  of  the  property  can  be  secured 
and  the  lines  are  marked  with  sufficient  dis- 
tinctness on  the  ground,  the  points  of  the  sur- 
vey may  be  tied  in  by  reference  to  such  lines. 
If  no  such  marks  exist  on  the  ground,  an  arbi- 
trary base  line  must  be  established  (and  perma- 
nently marked)  by  reference  to  which  the  sur- 
vey points  can  easily  be  tied  in  and  platted. 
The  plat  must  show  the  development  work,  if 
any  has  been  done,  the  geological  formations, 
topography,  and  all  work  of  the  survey. 

Lastly— and  this  is  the  object  of  the  survey— 
the  engineer  must  indicate  on  the  plat  the  loca- 
tion and  extent  of  all  ore-deposits  in  the  sur- 
vey field,  whether  exposed  in  part  or  not  at  all. 
A  distinctive  color  should  be  used  to  display  ore- 
bodies  on  the  map,  so  that  no  mistaken  idea  can, 
by  any  chance,  be  conveyed. 

The  written  portion  of  the  report  should  give 
a  clear  description  of  the  conditions  found  and 
an  explanation  of  the  deductions  of  the  survey 
as  indicated  on  the  map.  A  clear  statement 
should  be  made  showing  just  what  facts  are 
determined  and  what  are  left  undetermined. 

Sec.  8.  Field  Notes.  In  the  field-book  a 
graphic  record  of  the  work  is  kept.  Sketches  in 
plan  and  section  are  used  to  show  the  location 


of  all  survey  points  and  are  much  to  be  pre- 
ferred to  any  system  of  notes. 

All  readings  of  the  instrument  must  be  re- 
corded, even  those  taken  on  the  test  of  each 
earth-contact.  The  field  book  must  show  every 
detail  of  the  work,  no  matter  of  how  small  im- 
portance it  may  appear  to  be,  recorded  in  such 
form  that  any  other  engineer  could,  from  the 
notes,  repeat  the  work  exactly  as  done,  any  time 
thereafter. 

CHAPTER  V. 
APPLICATION  OF  THE  TERREOHMETRIC  ART. 

Sec.  1.  General  Principles.  The  first  step  in 
a  terreohmetric  survey  is  to  make  a  study  of  the 
conditions,  get  an  understanding  of  the  problem 
to  be  solved,  then  form  a  general  plan  of 
operation. 

All  available  information  should  be  collected 
relating  to  the  character,  extent  and  mode  of 
occurrence  of  ore  bodies  on  the  field  or  in  the 
neighborhood.  While  it  is  to  be  remembered  that 
ore  bodies  never  occur  twice  exactly  alike,  yet 
it  is  one  of  the  first  principles  by  which  the  or- 
dinary mining  expert  exerts  his  judgment,  that 
wherever,  as  far  as  is  shown  by  development 
work,  a  similarity  of  conditions  exists  in  differ- 
ent places,  especially  if  they  be  in  the  same 
neighborhood,  a  complete  similarity  with  ref- 
erence to  undeveloped  features  may  be  supposed 
to  exist. 

This  rule  might  assume  a  more  absolute  nature 
if  all  the  conditions — not  only  of  present  state, 
but  of  geological  origin  and  the  influences  that 

29 


have  wrought  perhaps  many  intermediate 
changes — could  be  fully  known.  But  even  the 
present  state  of  conditions  must  often  be  judged 
by  necessarily  incomplete  tests,  and  the  rest  is 
determined — if  at  all — still  less  definitely;  con- 
sequently, the  expert  is  often  puzzled  and  gives 
faulty  advice,  having  little  to  base  his  judgment 
upon.  But  his  method  is  nevertheless  truly 
scientific. 

The  terreohmetric  engineer  should  look  the 
field  over,  applying  the  knowledge  of  the  ordi- 
nary mining  expert  as  far  as  he  is  able;  then, 
as  a  further  application  of  the  same  method, 
use  terreohmetry  to  accumulate  a  mass  of  data 
that  throws  a  flood  of  additional  light  on  the 
problem. 

Where '  bodies  of  mineral  or  ore-bearing 
ledges  are  sufficiently  exposed,  either  naturally 
or  by  development  work,  so  that  their  real  con- 
dition is  revealed,  preliminary  measurements  are 
taken  through  such  known  conditions,  in  order  to 
establish  a  standard,  as  it  were,  by  which  UBH 
known  conditions  in  the  same  vicinity  may  be 
judged  when  measured.  Such  test  measure* 
ments  must  be  short  enough  to  preclude  the  pos- 
sibility of  some  as  yet  unknown  ore-body  being 
reached  by  the  current  and  utilized  as  a  partial 
conductor.  (See  Sec.  5.) 

Wherever  possible  a  known  condition  should 
be  used  as  a  starting  point  for  a  survey.  This 
is  not  essential  but  is  often  a  valuable  aid  and 
should  not  be  neglected  if  practicable. 

Sec.  2.  Arcs,  or  the  Method  of  Locating 
Ores  Occurring  in  more  or  less  Horizontal 

30 


Beds  or  Irregular  Deposits.  The  simplest  plan, 
of  survey  is  that  in  which  arcs  alone  are  employ- 
ed. (See  Fig.  6.)  This  plan  has  been  used  in 
the  Missouri  lead  and  zinc  fields,  and  is  espe- 
cially applicable  to  such  conditions  as  there 
found;  namely,  the  occurrence  of  ore  bodies  in 
irregular  masses  at  no  great  depth  from  the 
surface. 


FI6.6 


A  station,  as  A,  is  established  upon  or  near 
the  area  to  be  surveyed.  From  point  A  as  a 
center,  with  radius  of,  say  350  feet,  an  arc  of  a 
circumference,  as  EF,  is  described.  Upon  this 
line  at  points  20  feet  apart,  contacts  are  succes- 
sively made  and  readings  taken  to  station  A. 
A  comparison  of  these  readings  shows  a  line  or 
area  of  low  resistance.  This  operation  is  re- 
peated with  another  arc  from  station  B,  by 
which  it  is  shown  that  under  point  X  an  ore 
body  exists. 

Successive  stations  are  used  until  the  entire 

31 


area  it  is  desired  to  survey  has  been  covered. 
The  arcs  should  be  made  to  overlap  sufficiently 
to  make  sure  of  locating  a  mineral  body  any- 
where in  the  field. 


Sec.  3.  Parallels,  or  the  Method  of  Locat- 
ing Ledges.  To  trace  up  float  or  locate 
any  blind  ledge,  proceed  as  follows :  ( See  Fig.  7. ) 
Lay  out  two  parallel  lines,  as  jk  and  mn,  at  a 
distance  of,  say,  300  feet  apart,  (See  Sec.  5.) 
and  at  right  angles  to  the  direction  it  is  sup- 
posed the  ledge  strikes. 

Establish  Station  A  on  line  j  k.  Take  meas- 
urements from  points,  as  n,  s,  t,  u,  etc.,  on  line 
mn  (which  points  are  from  12  to  16  feet  apart) 
to  station  A.  No  variation*  being  found  within 

*Note. — The  term  variation  has  an  important  use 
in  terreohmetry.  It  applies  to  the  comparison  exist- 
ing between  the  measurements  of  a  set,  as,  those 
from  station  A  to  points  on  arc  EF  in  fig.  6,  or  from 
station  A  to  points  on  line  n  B  in  fig.  7,  or  from 
station  A  to  points  on  line  m  n  in  fig.  8 ;  or  gener- 
ally, any  proper  comparison  between  measurements. 
See  sec.  16. 

82 


a  distance  of,  say,  100  feet  from  n,  station  B  is 
established  and  measurements  are  taken  thereto 
from  points  on  line  j  k,  as  indicated  on  the  fig- 
ure. Again,  no  variation  being  found,  station  C 
is  established  and  the  operation  repeated.  This 
time  a  strong  variation  is  shown  with  point  of 
lowest  resistance  at  D,  where  a  station  is  estab- 
lished and  a  fourth  set  of  readings  shows  a  line 
of  low  resistance  passing  through  D  and  E. 


FIG.8 


Sec.  4.  Tracing  Extended  Deposits.  Any 
ledge,  the  location  of  which  is  already  partly 
known  by  development  work,  or  by  the  process 
of  location  just  described,  may  be  traced  and 
definitely  located  throughout  its  entire  length  by 
the  process  illustrated  in  figure  8. 

Station  A  is  established  in  or  over  the  known 
portion.  A  series  of  straight  lines,  as  m  n,  o  p, 
q  r,  etc.,  are  used,  in  each  of  which  a  point  of 
lowest  resistance  is  found.  These  points  (B,  C,  D, 
etc. )  are  used  successively  as  stations,  as  indicated 
in  the  illustration.  On  line  uv,  two  points  of 
low  resistance  are  found  showing  that  the  ledge 
splits. 

The  distance  between  the  lines  m  n,  o  p,  etc., 
may  be  less  than  50  feet,  or  it  may  be  as  much 
as  400  or  500  feet,  or  over,  depending  on  the 
conditions  of  depth  to  the  ore  and  whether  or 
not  it  is  desirable  to  make  any  determinations 

83 


with  regard  to  the  character  of  the  ledge  itself, 
as  is  further  explained  in  sections  5  and  6. 

The  lines  m  n,  op,  etc.,  should  be  laid  out, 
as  near  as  may  be  at  right  angles  to  the  strike  of 
the  ledge,  and  should  be  straight  lines,  not 
curves,  for  the  reason,  as  illustrated  in  fig.  9, 


that  the  current  does  not  traverse  the  shortest 
distance  between  the  points  of  contact  as  A 
and  m,  but  travels  by  the  line  of  least  resistance. 
In  other  words,  from  any  point  of  contact  not 
in  the  ledge,  as  m,  the  current  traverses  country 
rock  to  the  ledge  by  the  shortest  route,  thence 
along  the  ledge  to  the  other  point  of  contact, 
as  A. 

As  the  distance  of  the  contact  from  the  ledge, 
as  mB,  is  successively  shortened  or  lengthened 
by  moving  the  contact  in  either  direction  along 
line  m  n,  as  from  m  to  o  or  from  o  to  m,  the 
length  of  earth  section  traversed  by  the  current 
is  successively  decreased  or  increased  by  the  dis- 
tance m  o,  and  the  resistance  is  also  decreased 
or  increased,  but  in  a  greater  ratio  than  that  of 
distance,  for  the  reason  that  points  o,  m,  etc., 
are  successively  removed  from  the  center  of  the 
zone  of  more  or  less  mineralized  material  which 
surrounds  every  mineral  body  in  the  earth,  and 
therefore,  at  each  successive  receding  move,  a 

34 


portion  of  Jess  mineralized,  higher  resistance  ma- 
terial is  included. 

When  the  contact  point  is  so  far  removed  from 
the  ledge  that  a  section  of  practically  non-min- 
eral earth  is  included,  no  matter  how  short  such 
section  may  be,  the  resistance  reaches  a  high 
point  at  once.  (See  Chap.  I,  Sec.  4.) 


FIS.ZO.       A  LONGITUDINAL  SECTION 


Sec.  5.  Determining  Depth.  The  location 
of  mineral  bodies  with  relation  to  their  depth 
from  the  surface  is  determined  by  the  method 
illustrated  in  fig.  10.  The  condition  therein  rep- 
resented is  a  ledge,  covered  by  drift,  which  it 
is  assumed  has  been  traced  in  the  manner  de- 
scribed in  the  previous  section,  and  its  location 
in  plan  marked  on  the  surface. 

Earth-contacts  at  points  A  and  B  were  used 
in  that  operation  arid  the  record  of  the  work 
shows  that,  on  the  set  of  measurements  of  which 
point  B  was  finally  selected  as  the  lowest  ( as  those 
taken  on  line  m  n  in  fig.  8)  there  was  a  consider- 
able variation.  That  is,  the  resistance  from  A 
to  B  was  small  compared  with  that  from  A  to  m 
and  other  points  on  line  m  n. 

This  variation  is  due  to  the  fact  that  the  cur- 
rent, in  traversing  the  earth  between  points  A 
and  B  (see  Fig.  10),  finds  its  path  of  least  resist- 

86 


amee  through  the  high-resistance  drift,  perpen- 
dicularly from  both  contacts  to  the  low  resist- 
ance mineral  body,  as  indicated  by  the  shaded 
portions  of  the  figure,  thence  laterally  through 
the  mineral  between  x  and  z;  but  when  earth- 
contact  B  is  removed  along  line  m  n  (see  Fig. 
11)  to  a  point,  as  m,  some  distance  from  the 


.II.     R055  SECTION 


ledge,  the  current  is  obliged  to  traverse  an  added 
portion  of  high  resistance  material,  for  the  dis- 
tance from  m  to  z  is  much  greater  than  from  B 
to  z,  and  probably  includes  material  of  much 
higher  resistance,  as  m  is  further  removed  from 
the  more  or  less  mineralized  zone.  (See  Sec.  4.) 

The  first  move  in  the  operation  of  determining 
the  depth  is  to  shorten  the  earth  section  by  re- 
moving the  earth-contact  from  B  to  C.  (See  Fig. 
10.)  A  set  of  measurements  is  taken  from  A  to 
points  on  a  line  projected  through  C  at  right 
angles  to  the  strike  of  the  ledge,  to  determine 
what  degree  -of  variation  exists.  It  is  found  to 
be  considerable. 

The  distance  is  again  shortened  by  abandon- 
ing contact  C  and  making  a  contact  at  D.  The 
effect  now  is  changed.  The  current  in  passing 
between  A  and  D  finds  its  path  of  least  resist- 
ance near  the  surface  as  indicated  by  the  shad- 
ing in  the  figure,  and  does  not  reach  the  mineral 


• 


V    or 


body.  Consequently,  when  contact  is  made  at 
successive  points  on  a  line  through  D  at  right 
angles  to  the  ledge,  the  set  of  measurements 
from  A  to  such  successive  points  shows  no  varia- 
tion. The  depth  is  now  known  to  be  approxi- 
mately one-half  the  distance  A  D. 

Thus  the  depth  is  determined  by  successively 
shortening  the  length  of  earth  section  employed 
until  the  variation  is  lost.  The  depth  is  equal, 
then,  approximately  to  one-half  the  last  distance 
employed. 

Care  must  be  used  in  applying  this  principle 
to  make  allowance  for  modifying  conditions. 
For  instance,  if  a  strongly  mineralized  zone  over- 
lies the  mineral,  the  position  of  the  upper  por- 
tion of  this  zone  will  be  determined,  not  that  of 
the  mineral  itself. 

Where  the  condition  is  that  illustrated  in  fig- 
ure 6,  the  successive  shortenings  must  be  at  both 
ends  of  the  earth  section  simultaneously,  so  as 
to  keep  the  middle  of  the  earth  section  employed 
approximately  over  the  center  of  the  mineral 
body. 

From  a  consideration  of  this  principle  it  will 
be  seen  that  the  depth  to  which  the  field  is  pros- 
pected by  the  survey  depends  upon  the  length 
of  earth  sections  measured,  and  the  latter  should 
be  at  least  twice  the  depth  it  is  desired  to  reach. 

It  must  be  remembered  that  this  principle  of 
depth  determination  applies  as  well  to  lateral 
conditions  as  to  vertical ;  or,  in  other  words, 
that  by  this  means  it  may  be  known  to  what  ex- 
tent the  current  deviates  from  a  straight  line 
laterally  as  well  as  vertically. 

Always  commence  by  measuring     long     dis- 

37 


tances,  thus  learning  the  general  features  of  the 
field.  Afterwards  fill  in  the  details  with  meas- 
urements of  appropriate  length,  together  with 
the  use  of  such  stations  as  may  seem  proper  in 
the  light  of  preliminary  determinations. 

Sec.  6.  Comparison  of  Different  Portions  of 
Deposit.  To  secure  a  comparison  of  different 
sections  of  the  same  ledge  proceed  as  follows: 
(See  Fig.  8.)  Tabulate  so  as  to  easily  compare 
measurements  A  B,  B  C,  CD,  D  E,  E  F,  etc.; 
also  A  C,  B  D,  C  E,  D  F,  etc. ;  also  AD,  BE, 
C  F,  etc ;  also  A  E,  B  F,  etc.  The  variation  in 
each  of  these  sets,  or  tables,  shows  a  correspond- 
ing variation  either  in  the  depth  of  drift  over- 
lying the  ledge,  or  in  the  mass  (or  width)  of 
material  having  the  same  resistance,  or  in  the 
mineral  content  of  the  ledge,  or  there  may  be 
any  combination  of  these  causes.  Often  the 
depth  of  overlying  drift  can  be  easily  determin- 
ed, when  proper  allowance  may  be  made,  thus 
eliminating  this  unknown  factor  from  the  prob- 
lem. Tests  by  development  work  may  then  easily 
be  made  to  determine  the  exact  cause  of  varia- 
tion. 

By  this  means  the  location  of  the  apex  and 
direction  of  trend  of  an  ore  chute  in  a  compara- 
tively barren  ledge  may  be  determined.  Various 
other  applications  of  this  principle  may  be  made. 
Care  must  be  used  to  observe  the  conditions  ac- 
curately so  that  correct  deductions  shall  be 
made. 

Sec.  7.  Size  of  Deposit.  A  slide  or  isolated 
pocket  may  easily  be  distinguished  from  a  con- 
tinuous ledge  in  place  by  the  fact  that,  while  a 
large  variation  is  shown  by  a  set  of  short  moas- 

38 


urements  which  include  it,  the  mass  and  extent 
of  the  mineral  are  only  sufficient  to  produce  a 
slight  variation  on  longer  measurements. 

Sec.  8.  Faults  are  detected  by  the  process 
of  tracing  described  in  section  4.  Where  a  ledge 
has  been  "lost"  by  reason  of  a  fault,  the  lost 
portion  may  be  located  either  as  a  separate  ledge 
by  the  system  described  in  section  3,  or  a  sta- 
tion may  be  placed  in  the  mineral  body  at  a 
point  near  the  fault  and  sets  of  measurements 
taken  from  this  station  to  lines  of  points  on  the 
surface  as  in  tracing  a  ledge. 

Sec.  9.  Complicated  Problems  are  created  by 
the  presence  in  the  field  of  a  number  of  prac- 
tically separate  mineral  bodies  which  may  over- 
lie one  another  or  occur  in  such  position  that 
one  only  is  apparently  shown  by  the  survey 
when  there  are  really  two;  by  the  presence  of 
different  kinds  of  mineral  bodies  either  associ- 
ated together  or  occurring  in  close  proximity; 
or  by  the  occurrence  of  parallel  or  connected 
ledges.  The  fact  that  such  problems  are  more 
or  less  complicated  means  simply  that  a  greater 
degree  of  care  and  patience  must  be  exercised 
with  them  than!  where  simpler  conditions  pre- 
vail. 

It  should  be  noted  that  a  single  station  placed 
in  an  extensive,  but  continuous,  mineral  body 
usually  serves  all  the  purpose  that  could  be 
served  by  a  number  of  stations  placed  in  differ- 
ent parts  thereof,  for  the  resistance  through 
long  or  short  sections  of  continuous  mineral  bod- 
ies is  usually  very  small. 

Sec.  10.  Placer.  The  pay  streak  in  placer 
ground  may  be  located  and  traced  by  the  same 

39 


systems  as  those  used  for  ledges,  (see  Seca 
3  and  4)  where  the  conditions  are  such  that  the 
current  can  be  made  to  reach  the  metalliferous 
gravel  or  sand  without  danger  of  being  diverted, 
in  whole  or  in  part,  to  some  other  low-resistantce 
material  lying  or  extending  above,  below,  at  one 
side  of,  or  across  the  pay  streak.  To  avoid  the 
possibility  of  such  diversion,  the  measurements 
should  generally  be  made  through  the  shortest 
earth  sections  that  can  be  used  consistent  with 
the  depth  necessary  to  be  reached.  (See  Sec. 
5.) 

The  possible  presence  of  mineral  in  placer 
form  in  the  bed  of  every  gulch  and  canyon  as 
well  as  elsewhere  must  not  be  forgotten,  as  it 
often  has  a  considerable  influence  upon  measure- 
ments that  are  intended  for  quite  another  pur- 
pose than  the  determination  of  its  presence.  A 
few  test  measurements  will  reveal  its  presence 
or  absence  in  any  given  locality.  The  fact  thus 
determined  can  then  be  taken  into  consideration 
if  necessary  when  making  deductions  from  a 
comparison  of  measurements  through  earth  so 
affected  with  measurements  through  earth  not 
so  affected. 

Where,  in  the  course  of  a  survey,  a  set  of 
measurements  is  taken  across  a  strip  of  placer 
ground,  it  may  be  assumed — unless  there  is  rea- 
son to  believe  otherwise,  when  proper  tests  must 
be  made  to  secure  a  determination  of  fact — that 
the  same  effect  is  had  upon  each  measurement, 
-so  that  no  correction  need  be  made.  This  prin- 
ciple holds  good  where  the  ore  dump  of  a  mine 
or  the  tail  race  from  a  mill  are  included  so  as 
to  become  possible  factors.  It  has  been  noted 

40 


in  practice  that  these  latter  have  little  effect 
upon  earth  measurements  if  neither  contact  is 
made  near  to  them.  It  is  not  wise  to  establish 
a  station  in  or  very  near  to  a  tail  race  or  large 
ore  dump. 

Sec.  11.  Massive  Minerals.  Such  minerals  as 
Silver,  lead,  copper,  zinc,  etc.,  that  usually  occur 
in  large  masses,  present  the  simplest  problems 
to  the  engineer.  In  whatever  form  they  appear, 
but  especially  in  the  sulphide,  they  offer  very 
small  resistance. 

Sec.  12..  Gold  stands  high  in  the  scale  of 
specific  conductivity  and  is  usually  so  intimately 
associated  with  other  metals  that  the  total  metal- 
liferous mass  diffused  through  the  gangue  rock 
is  sufficient  to  render  the  problem  of  its  loca- 
tion easily  solved.  Even  where  it  is  found  al- 
most wholly  free  from  association  with  other 
minerals,  if  encased  in  a  high-resistance  country 
rock,  it  may  still  be  easily  located.  (See  Chap. 
I,  Sec.  2.) 

Sec.  13.  Buried  Treasure,  if  composed  of 
metal  or  encased  in  a  metal  box,  buried  in  earth 
without  wrappings  which  will  act  as  insulators, 
may  easily  be  located,  provided  the  earth  be 
quite  damp  or  that  it  has  been  soaked  by  rain 
or  otherwise  at  any  time  after  the  treasure  was 
buried.  If  a  bag  or  wrappings  of  any  kind  were 
originally  used,  or  if  metal  were  encased  in  a 
wooden  box,  the  metallic  object  may  still  be  lo- 
cated if  sufficient  time  has  elapsed  for  the  bag 
or  box  to  become  so  rotted  and  penetrated  with 
earth  moisture  that  the  metal  is  practically  in 
contact  with  the  earth. 

It  is  usually  best  to  use  the  plan  of  operation 

41 


described  in  section  2.  The  length  of  radius 
should  be,  say,  15  to  25  feet,  but  never  less  than 
twice  the  supposed  depth  from  the  surface,  of 
the  object  sought.  If  the  object  be  small  in  mass 
and  buried  quite  deep,  say  10  feet  or  more,  it 
is  a  good  plan  to  use  rods  six  to  eight  feet  long 
for  making  contacts,  especially  if  the  ground  be 
soft.  "When  siuch  long  rods  are  used  the  radius 
may  be  shortened  accordingly.  A  short  radius 
is  preferable  to  a  long  one  in  such  work,  as  its 
use  produces  a  larger  variation-  when  the  earth 
section  is  measured  which  includes  the  small 
metal  object,  and  so  renders  the  location  of  such 
object  more  certain  and  easy. 

As  an  example  -of  actual  work  of  this  kind, 
in  one  instance  a  silver  dollar  was  known  to 
be  buried  a  few  inches  deep.  Measurements 
through  two  feet  of  earth  were  taken  with  rods 
inserted  one  foot  deep  and  showed  resistance  of 
from  30  to  35  ohms.  When  the  right  spot  was 
reached  so  that  the  coin  was  included,  the  re- 
sistance measured  only  15  ohma 

Sec.  14.  Problems  Impossible  To  Solve.  Wa- 
ter, oil,  coal,  sulphur,  precious  stones,  and  all 
such  substances  that  are  not  better  conductors 
of  electricity  than  the  rocks  with  which  nature 
envelopes  them,  cannot  be  located  by  terre- 
ohmetry. 

Sec.  15.  Country  Rocks  of  Low  Resistance. 
Clay  and  talc  in  their  various  forms  consist 
largely  of  aluminum,  which  stands  high  in  the 
scale  of  conductivity.  Consequently  these  sub- 
stances, when  massive,  offer  lower  resistance 
than  other  adjacent  country  rocks,  and  their 
presence  sometimes  interferes  with  the  location 

42 


of  minerals.  When  they  are  present,  tests  of 
their  resistance  should  be  made  and  the  condi- 
tions of  their  occurrence  carefully  noted,  so  that 
the  facts  with  relation  to  them  may  be  known 
and  taken  into  consideration  at.  all  times  on  the 
survey. 

Sec.  16.  How  to  Make  Deductions.  Meas- 
urements of  resistance  may  be  compared  and  the 
variation  noted  (1)  when  from  the  same  station 
and  of  the  same  length,  (2)  when  of  the  same 
length  though  from  different  stations,  (3)  when 
of  different  lengths  and  from  the  same  or  differ- 
ent stations.  In  the  first  and  second  cases,  the 
variation  shows  at  once  the  comparative  conduc- 
tivity of  the  earth  sections  and  by  this  their 
mineral  content  may  be  judged.  Careful  discrim- 
ination must  be  used  to  note  the  true  meaning  of 
variations,  especially  in  the  third  case.  Very 
often  valuable  deductions  can  be  made  from 
such  comparisons,  but  the  work  should  always 
be  platted  and  carefully  studied  in  view  of  the 
geological  and  topographical  conditions  in  order 
to  see  the  meaning  of  the  results  with  correct- 
ness and  certainty. 

The  different  degrees  of  variation  have  not 
always  the  same  meaning.  If  the  variation  is 
in  the  ratio  of  1  to  10  over  a  ledge  in  one  place 
and  in  the  ratio  of  1  to  4  in  another  place,  it 
may  mean  that  the  ledge  bears  less  mineral  in 
the  second  place  or  that  it  is  buried  to  a  greater 
depth  beneath  drift;  while  a  variation  in  the 
ratio  of  1  to  10  again  in  a  third  place  does  not 
necessarily  mean  that  the  same  condition  exists 
as  at  the  first  point.  For  the  result  noted  may 
be  caused  by  the  occurrence  of  a  larger  mass 

43 


of  lower  grade  ore,  or  the  presence  of  a  dif- 
ferent mineral,  or  a  change  in  the  country  rock. 
These  points  must  be  kept  in  mind  and  tests 
made  to  determine  just  what  are  the  causes  of 
the  results  secured,  even  if  such  tests  must  be 
made  with  the  pick  and  shovel. 

The  general  plan  of  the  survey  should  be  made 
with  the  idea  of  producing  results  that  have  a 
definite,  decided  meaning.  It  is  no  use  to  do 
work  if  the  meaning  of  its  results  cannot  be 
•definitely  determined.  However  such  determina- 
tions often  must  be  made  by  development  and 
the  work  is  none  the  less  valuable  on  that  ac- 
count. 

As  one  who  is  seeking  for  hidden  treasure 
is  sometimes  disappointed  by  digging  up  tin 
cans  or  old  iron,  so  the  miner  occasionally  finds 
a  deposit  of  i^on  sulphurets  when  he  looks  for 
something  more  valuable. 

Too  much  should  not  be  expected  from  a  sur- 
vey, but  it  nearly  always  furnishes  at  least  some 
information  of  great  value— often  very  much. 
The  exercise  of  intelligence  and  care  are  neces- 
sary at  every  step.  It  is  impossible  to  give  more 
explicit  directions  for  conducting  a  survey,  be- 
cause conditions  are  never  alike  in  two  places; 
but  the  fundamental  principles  have  been  ex- 
plained and  these  may  be  applied  either  singly  or 
in  various  combinations  as  the  conditions  of 
each  problem  seem  to  require  in  the  estimation 
of  the  engineer. 


THE 

^  UNIVERSITY  ) 

•££S*LORH\h. 


YA  01719 


